"Something went wrong. Try clicking Add block again." Error on Dawn 2.0 Theme

[gov]

I am trying to test Theme App Extensions.

I am using the code from https://github.com/Shopify/theme-extension-getting-started

I was able to push via `shopify extension push` and enabled preview mode.

However when trying to play with the extension in the theme editor I just see the error: 

Appreciate your help.